1. Field of the Invention
The present invention relates to an optical module and its assembling method, and more specifically, to an optical module and its assembling method of a projecting apparatus.
2. Description of the Prior Art
A projecting apparatus for generating an image is disclosed in U.S. Pat. No. 6,089,719 xe2x80x9cProjecting Apparatus For Displaying Electrical Imagesxe2x80x9d. Please refer to FIG. 1 showing the projecting apparatus 10 according to U.S. Pat. No. 6,089,719. To display an image on a screen 34, a projecting apparatus 10 comprised of a light source device 22, three modulating units 24, 26, 28, a dichroic-polarization beam splitter prism 30 composed of four triangular prisms 36 of equal proportion, and a projecting lens 32 is used. The light source device 22 is used to generate three different-colored rays red, green, and blue with uniform illumination but different polarities. Each of the modulating units 24, 26, 28 modulates and changes the polarity of one of the single-colored polarized raysby means of reflection. The beam splitter prism 30 is used to first receive the polarized rays of red, green and blue, then guide the rays to their respective modulating units 24, 26, 28 for modulation, and finally converge the three modulated rays into an output beam all of which are accomplished via mirror planes, which transmit or reflect light based its polarity, plated onto the triangular prisms 36 of the beam splitter prism 30. The projecting lens 32 is installed in front of the output face of the beam splitter prism 30 for projecting the output beam to a screen 34.
Other projecting apparatus arealso disclosed in U.S. Pat. No. 6,247,814 xe2x80x9cProjecting Apparatus For Displaying Electronic Imagesxe2x80x9d and U.S. Pat. No. 6,364,488 xe2x80x9cProjection Display Device For Displaying Electrically Encoded Imagesxe2x80x9d thatincorporate the use of an L-shaped optical module to create an optical path of approximately the same length for the three monochrome rays red, green, and blue in order to reduce the optical design of the projecting apparatus. Please refer to FIG. 2 showing the projecting apparatus 40 according to U.S. Pat. No. 6,247,814. The projecting apparatus 40 includes a light source 42, three modulating units 44, 46, 48, an L-shaped optical module 50, an input lens set 52 and a projecting lens 54.
The light source 42 is for generating monochrome rays in red, green and blue in the same polarity. The three modulating units 44, 46, 48 are for modulating a single-colored polarized ray and changing its polarity by manner of reflection. The L-shaped optical module 50 is for controlling the path of each single-colored polarized ray. The input lens set 52 is installed between the light source 42 and the inner side of the L-shaped optical module 50. The projecting lens 54 is for projecting the beam output from the L-shaped optical module 50 to a screen 56.
To elaborate further upon the L-shaped optical 50, its makeup consists of three rectangular, transparent light-guide units, which are named respectively as a first, second, and third light-guide unit 60, 62, 64. Each light-guide unit is composed of a mirror sandwiched between the diagonals of two triangular prisms 66. The first and third light-guide units 60,64 have a polarization light beam splitter mirror 70,74 respectively while the second light-guide unit has a dichroic mirror 72.
The arrangement of the light-guide units has the second light-guide unit 62 restingat the apexof the first and the third light-guide units 60, 64. This arrangement ideally causes the first and the third polarization beam splitter mirror 70, 74 to be aligned along the same plane, and the second dichroic mirror 72 to be perpendicular to both the first and the third polarization beam splitter mirrors 70, 74. Light is input through the right angle that is located on the inside of the L-shaped optical module 50 and formed by of perpendicular sides 61, 65 of the first and the third light-guide units 60, 64.
Please refer to FIG. 3 now to follow how a projecting apparatus 40 operates. Generally, image signals wish to be displayedare input into the projecting apparatus 40 where images corresponding to the input signals are generated. For instance, the signal from the output port of a computers video card can be connected to the projecting apparatus 40 in order to display the operational mode of the computer. The three modulating units 44, 46, 48 of the projecting apparatus 40 each modulate their respective monochromatic beam according to received image signals. Then an image from each monochromatic beam (a red image 12, a green image 14 and a blue image 16) is outputted and brought together to create one image for users to see.
Continuing with the example of displaying a computer operational mode, the three images (red image 12, the green image 14 and the blue image 16) have equal resolutions (e.g. 800*600 or 1024*768) composed from a plurality of pixels 18. This means that pixels 18 from each of the three images with the same coordinates all correspond to one another. Under ideal conditions, the angles at which the red image 12, the green image 14 and the blue image 16 are projected onto the screen 56 are less than the maximum tolerance level, resulting in the overlap of pixels from the three images with the same coordinates at the same position.
For instance, if the projection angles of the red image 12, the green image 14 and the blue image 16 on the screen 56 are each less than the maximum tolerance level, a pixel 20R on the upper left corner of the red image 12, a pixel 20G on the upper left corner of the green image 14, and a pixel 20B on the upper left corner of the red image 16 will overlap one another and form a single pixel for users to see. However, if any of the projection angles of the red image 12, the green image 14 and the blue image 16 onto the screen 56 is larger than the maximum tolerance level, the pixels of the images with larger than maximum tolerance levels will not be in-line thereby decreasing projection quality. Therefore, it is imperative that the projection angles of the red image 12, the green image 14 and the blue image 16 on the screen 56 each be made less than the maximum tolerance level.
The part that has the biggest effect on whether the projection angles fall within tolerance levels is the L-shaped optical 50 more specifically the three light-guide units 60, 62, 64. Reason being if the three light-guide units 60, 62, 64 are not in proper position with respect to one another, the light-guide units 60, 62, 64 will project their respective single-colored polarized rays at different angles resulting in image quality degradation. Therefore, the design of a conventional L-shaped optical module 50 usually incorporates a holder 80 (FIG. 4) to align and maintain the positions of the three light-guide units 60, 62, 64 so as to guarantee the paths of the single-colored polarized rays.
However, the current-conventional method for assembling the L-shaped optical module 50 is not ideal because it easily leads to misalignment of parts. As can be deduced from the above-given information, any small misalignment can cause any, two, or all of the three beams of red, blue and green to be projected at angles above the maximum tolerance. Pixels therefore do not overlap but lie on different positions on the screen, resultingin lower than expected image quality.
Under conventional methods the manufacturing of light-guide units 60, 62, 64 involves gluing a mirror between two prisms, the mirror of choice either a dichroic mirror 72 or a polarization beam splitter mirror 70,74 depending on the type of light-guide unit is being produced. The assembled light-guide units 60, 62, 64 in FIG. 2 are then attached to the holder 80 as shown in FIG. 4. Each light-guide unit 60, 62, 64 is glued toa different set of four points located on the holder 80. For instance, the second light-guide unit 62 is glued onto the set of points 82, 84, 86, 88 of holder 80, wherein one prism 66 is glued to points 82, 84 and the other prism 66 is glued to points 86, 88.
The problem lies in tolerated errors that occur during the manufacturing process. More specifically, when the two prisms 66 are being glued to one of the two mirrors, the bottoms of the three parts are not always perfectly level when glued together. This means that the assembled light-guide 60, 62, 64 will not lie flush with surface of the holder 80. The effect of such an error is shown in FIG. 5.
FIG. 5 shows a cross-sectional view of the optical module 50 along the line 5xe2x80x945 in FIG. 4 as viewed from the upper right corner. While FIG. 5 assumes that only one light-guide unit has been assembled with error, one, two, or all three may have the error illustrated by the figure. Because the two prisms 66 of the second light-guide unit 62 were not glued to the dichroic mirror 72 at an even level, the prism 66 glued to points 86, 88 rests higher than the prism 66 glued to points 82, 84. As a result,the dichroic mirror 72 lies at a slant causing the paths of the single-colored polarized rays in the L-shaped optical module 50 to deviate from the intended path. For instance, a green polarized ray G* is reflected by a modulating unit 44 and then passes through the first light-guide unit 60 to the second dichroic mirror 72. Because the dichroic mirror 72 lies at aslant, the polarized ray G* will deviate from the intended path. Therefore, due to the slight error in assembly of the conventional optical module 50, the projection angles of the red image 12, the green image 14 and the blue image 16 may possibly be larger than the maximum tolerance level, which causes low image quality in the optical module 50.
As stated before every manufacturing process has a tolerance level for errors, meaning that every part has slight imperfections. One imperfect part such as the example in FIG. 5 may or may not affect the projection angle enough to cause the angle to be above the maximum tolerance level. However, it is more likely that more than one part is imperfect. The culmination of errors from all parts with imperfections that fall within assembly tolerance increases the likelihood of the projection angles of the monochrome rays to be greater than the maximum tolerance level. In other words, even though the assembly error for every part falls within the manufacturing tolerance for error, there is no guarantee that the angles at which the monochrome rays are projected will fall under the maximum tolerance level.
In addition to the manufacturing problem, there is the problem of the effect of temperature. The temperature difference between the off/onstates of the projecting apparatus 40 can be up to a few tens of degrees (e.g. room temperature is 20xc2x0 C. while the projecting apparatus 40 is up to 50xc2x0 C. in operation). However, since the prisms 66 and the holder 80 are made of different materials (in this case glass and metal respectively), the two have different expansion coefficients meaning that the prisms 66 will contract or expand at a rate different from the rate the holder 80 contracts or expands when the projecting apparatus 40 is switched off or on. This effect leads to two things one being the two prisms 66 will push each other away and the other being the holder will push each prism individually up. The ultimate effect is the light-guide unit 60, 62, 64 will become misaligned.
Please refer to FIG. 6 diagramming the forces caused by expansion when temperature rises after the projecting apparatus 40 is switched on. As mentioned above, the prisms 66 are made of glass, and the holder 80 is made of metal. When the temperature rises, the holder 80 will exert forces of F1, F2, F3, F4 on the points 82, 84, 86, 88 to which the prisms 66 are glued because the holders 80 expansion coefficient is larger than that of the prisms 66. In addition, the two prisms 66 will exert forces F5, F6 pushing each other away. With the holder only applying the forces F1, F2, F3, F4 to only the points 82, 84, 86, 88 instead of to the whole surface of the prisms 66 coupled with the prisms 66 pushing each other away, the light-guide unit 62 will rotate because it as a whole experiences an unbalanced moment of force. The rotation of light-guide unit 62 causes the path of the single-colored polarized rays to be changed.
It is therefore a primary objective of the present invention to provide an optical module and a method of assembly for a projecting apparatus to solve the problems mentioned above.
Briefly summarized, an optical module includes three light-guide units and a holder. Each of the light-guide units is composed of a mirror plane used to reflect and transmit light sandwiched between two prisms. The holder is composed of three installation areas, one for each light-guide unit, and two frames, each located between two of the installation areas. That is to say, the first frame is located between the first and second installation area while the second frame is located between the second installation area and the third installation area.
One of the features of the present invention is that, a first plane of the first light-guide unit is attached to and glued to a first side of the first frame, a second plane of the second light-guide unit is attached to and glued to a second side of the first frame, and a third plane of the third light-guide unit is attached to and glued to a third side of the second frame.
These and other objectives of the present invention will no doubt become obvious to those of ordinary skill in the art after reading the following detailed description of the preferred embodiment that is illustrated in the various figures and drawings.